Remote control units for controlling functions of vehicles are well known. Currently, the original equipment for many vehicles includes a wireless transmitter for arming/disarming the car alarm and/or locking or unlocking the car doors. Furthermore, aftermarket systems are available including wireless transmitters and receivers for controlling these and other functions, such as energizing the car starter to start the engine, so that the car will be warm or cool by the time the driver arrives.
Remote control transmitters which control multiple vehicle functions typically are configured to transmit control signals on different frequencies or to produce differently coded or modulated radio signals, each signal identifying the controlled function. The driver selects the desired function by pressing the associated button on the transmitter keypad, and the transmitter responds by transmitting the appropriate signal and/or code. For security purposes, transmitters controlling only a single function also often modulate or code their signal for security purposes, so that the vehicle will ignore signals which are not modulated or coded with a predetermined code assigned to the vehicle.
A difficulty with these systems is that the driver specifically selects which function to activate, which can lead to security failures. For example, the driver may unintentionally unlock the doors to the vehicle after it has been started, when the driver is a relatively long distance from the vehicle. Under these conditions it is possible that the vehicle might be stolen before the driver arrives at the vehicle. This problem can be avoided by reducing the transmitter power so that it has a limited range, but this solution has the accompanying problem that the driver may wish to be able to activate some functions, e.g., starting the engine, from a great distance from the vehicle, e.g. while the driver is inside of a shopping mall and the vehicle is parked outside.